A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. The patterning device may be supported by a patterning device support. The pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer) that may be supported by a substrate table. Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
The lithographic apparatus may be provided with a magnetic device for providing a force in a reference direction between a first part of the magnetic device and a second part of the magnetic device. FIG. 2a depicts a known magnetic device for use in the lithographic apparatus. The magnetic device comprises a first magnetic part coupled to the first part and having a first magnetic polarization and a second magnetic part coupled to the second part and having a second magnetic polarization. The first magnetic part and the second magnetic part are configured to magnetically interact with each other. The first magnetic part exerts a first force on the second magnetic part and the second magnetic part exerts a second force on the first magnetic part. The first force and the second force may have opposite directions that are parallel to a reference direction. Note that magnetic polarization is a vector field, i.e. it has both direction and magnitude, therefore the addition ‘direction’ in relation to magnetic polarization is omitted. The first magnetic polarization is substantially parallel to the reference direction and may be directed in an upward direction (e.g., as indicated in the first magnetic part MP1 by the arrow). The second magnetic part may have a second magnetic polarization that is substantially perpendicular to the reference direction. The force between the first magnetic part MP1 and the second magnetic part MP2 may be used to compensate for a gravitational force. In such an embodiment the magnetic device may be suitable to be used as a so-called gravity compensator.
The first magnetic part MP1 may be connected to the substrate table to support against the gravitational force. The first magnetic part MP1 may be connected to a patterning device support (e.g., mask table MT) to support it against the gravitational force. The gravity compensator may be provided to other components of the lithographic apparatus (e.g., lens elements, metrology frame, stages and more in general each component that is passively and/or actively held at a desired position) to provide support against the gravitational force while, preferably, it also dynamically isolates the components of the lithographic apparatus from vibrations from the surrounding of the apparatus.
Electromagnetic devices (e.g., motors) in general can be characterized by a parameter known as ‘motor constant’. This constant defines a relation between an electromagnetic device input and an electromagnetic device output, and in case of a Lorentz actuator the electromagnetic device input is usually a current carried by an electrically conductive element (e.g., a coil CL) and the electromagnetic device output is usually an exerted force as function of the current. FIG. 2b depicts another known electromagnetic device including an electrically conductive element for use in a lithographic apparatus. Most of the time, the value of the motor constant is assumed to be a constant, explaining the name ‘motor constant’. Thus, when a person skilled in the art assumes the motor constant to be constant, he/she actually assumes the value of the motor constant to be the constant average portion. One of the possible causes for the variation of the value of the motor constant of a Lorentz actuator can be position dependency due to position dependent variations of the magnetic field strength and/or magnetization direction/orientation of the magnets.
The magnetic field caused by the magnetic interaction between the first magnetic part MP1 and the second magnetic part MP2 may extend outside the magnetic device and may disturb another part (e.g., another part of the lithographic apparatus). FIG. 3 depicts a graph of the strength of the magnetic field (in Tesla) outside the magnetic device of FIG. 2a at a fixed distance (DST) of 39 mm in z-direction above the gravity compensator (indicated as level LVL in FIG. 2a) and in dependency of the distance in R-direction (in millimeters) with respect to a center axis (being parallel to the reference direction) and extending through the center of the magnetic parts MP1. The maximum strength of the magnetic field outside the gravity compensator may be too high. Additionally the position dependency of the value of the motor constant of the magnetic device may also be too high.